Are there other fish, or just the Sailfin Tang?
Search for threads on treating Ich. This topic gets brought up almost daily here and other people have posted great links with great information on how to treat Ich. The short story is that if you want to do it right, there are two treatment choices...hyposallinity or copper. Both kill the problem and both are done in a QT tank. Other methods (LFS
medicines) only treat the problem. You want to "cure", not just treat.
I hate to sound like the Tang Police here, but a Sailfin Tang really needs a much larger tank. Most Tangs will not thrive in a 55g. Check out places like liveaquaria.com or bluezoo aquatics.com for fish info, tank size suggestions and compatability. You will see differences in size suggestions sometimes. I tend to stick with the larger. Smaller tanks cause stress. Stress weakens immunity. Weakened fish are more susceptible to disease and parasites. This is especially true for Tangs.